The invention relates to a process and to a metal-ceramic compound material or metal-ceramic substrate.
The term “direct bonding” refers to processes for bonding a metal with ceramic, i.e. for the manufacture of metal-ceramic compound material, by means of a surface layer of the metal functioning as a eutectic and produced by a reactive gas, for example a metal oxide layer, which during heating of the metal and ceramic to be connected (hereinafter “bonding components”) to a processing or bonding temperature (eutectic temperature) melts and produces the bond between the bonding components by interlacing the ceramic as a kind of solder during the subsequent cooling. The bonding temperature in this process is below the melting temperature of the metal. In particular also depending on the metal used, this bonding temperature is between approximately 714° C. (copper phosphorous) and 1820° C. (chromium oxygen).
In a known direct bonding process (DE 23 19 854/U.S. Pat. No. 3,766,634) the metal is first oxidized at a temperature below the bonding temperature in a protective gas atmosphere containing oxygen. Afterwards, the metal and the ceramic are heated to the bonding temperature. This is followed by cooling to room temperature. For the copper-oxygen system an oxygen content of 0.01-0.5 percent by volume (100 to 5000 ppm) is specified for the reactive atmosphere.
A more advanced direct bonding process (DE 26 338 69/U.S. Pat. No. 3,994,430) is also known in which the metal is treated or oxidized with the reactive gas in a separate step preceding bonding. After the metal and ceramic components to be bonded are placed together, they are heated to the bonding temperature in a furnace, in a protective gas atmosphere with an oxygen content of 0.01-0.05 percent by volume (100-5000 ppm).
Furthermore, a direct bonding process (DE 30 36 128) is known in which the bonding of copper and ceramic takes place in a vacuum furnace with an oxygen content between 0.001-0.1 mbar (approximately 1-100 ppm).
It has also been suggested (DE 32 04 167/U.S. Pat. No. 4,483,810) to execute the direct bonding process in a continuous or tunnel furnace in a protective gas atmosphere, whereby the oxygen content is set to 20-50 ppm through the dosed addition of oxygen to the protective gas, at a temperature between 960° C. and 1072° C.
In all of the above known processes the oxygen content of the protective gas atmosphere is far above an equilibrium oxygen content of the copper-oxygen system. From the literature it is known that the oxygen content of this system is on the order of 2.6-5 ppm in the temperature range in which the direct bonding or DCB process is executed.
For example, in “The Metallurgy of Copper”, Incra Series, Vol II, pp. 56, 60, a value of 2.704 is stated for logK of the reaction Cu+ 1/2 O2=Cu2O at a temperature of 1085° C. This corresponds to a fractional oxygen content of 3.9×10−6 Atm, which is approximately 3.9 ppm.
From “Electronic Equilibrium Examinations of the Copper-Oxygen System between 1065 and 1300° C.”, post-doctoral thesis by J. Osterwald, Berlin 1965, the following formula is known for the temperature dependence of the fractional oxygen content:log(pO2)=−(20970/T)+10.166, where PO2 is the fractional oxygen content in ppm and T is the temperature in ° C.
From this formula the following fractional oxygen contents can be defined in dependency on the temperature:
Temperature in ° C.Fractional content in ppm10653.110754.210855.3
In a further publication (Neumann et al Metal Process, 1985, page 85) the oxygen content of the copper-oxygen system at a eutectic temperature of 1065° C. is specified as 2.69×10−6, which is approximately 2.69 ppm.
Therefore, it can be assumed that the equilibrium oxygen content in the direct bonding of copper and ceramic (DCB process) is between 2 and 6 ppm.
One of the disadvantages of the known processes mentioned above is therefore that due to the considerably higher oxygen content in the protective ga atmosphere in which the DCB process is conducted, post-oxidation of the copper takes place.
The present invention is based on the knowledge that the regulation of the oxygen content in a protective gas atmosphere in the range below 10 ppm, especially in the factory production of metal-ceramic composites or substrates is not possible with the required precision, or at most at a very high cost due to the complex technology involved. Furthermore, the invention is based on the knowledge that a fractional oxygen pressure or oxygen content in the protective gas atmosphere of the DCB process smaller than the equilibrium pressure of the metal (copper)-oxygen system leads to a reduction of the adhesion of the metal (copper) to the ceramic, while excessive fractional oxygen pressure or oxygen content in the protective gas atmosphere causes heavy post-oxidation, which in extreme cases can result in the melting of the entire metal, for example of the entire metal or copper foil.
The object of the present invention is to present a process which avoids the disadvantages of the known processes and which enables a simplified method of manufacturing metal-ceramic composites, especially metal-ceramic substrates and especially such substrates for electric or electronic circuits of extremely high quality.